In recent years, electro-photographic image forming apparatuses have been used in the field of shortrun printing, in which the quality of printed images and operating stability of the apparatus required for such apparatuses, have become very higher and much severer. Further, a large number of technologies have been proposed, by which the printed images exhibiting such high quality can be offered stably for long periods.
In the above usage, a new problem concerning the printed images has been realized, which is uneven reflection density exhibiting longitudinal streaks.
Said streaks will now be detailed.
If an original document, exhibiting various reflection densities of the image in the lateral direction perpendicular to a moving direction of an image carrier (which is a photoconductor), is continuously printed on recording sheets, and if a subsequent original document, carrying an even half-tone image over its total imaging area, is printed on a recording sheet, the above-described streaks are observed on said recording sheet, which are corresponding to the lateral distribution of the reflection density of said original document. Further, the higher the number of continuous prints increases, the more said uneven reflection density tends to occur.
After studying possible reasons of the occurrence of said uneven reflection density, the inventors presumed the occurrence mechanism described below.
The presumed occurrence mechanism will now be detailed.
Finding 1. When not transferred toner particles, remaining on the photoconductor, are removed from the surface of the photoconductor by an edge of a cleaning blade, the surface of the photoconductor is mechanically abraded.
Finding 2. Concerning said abrasion, occurred on the surface of the photoconductor due to the edge of the cleaning blade, the more the amount of the not transferred toner particles, supplied to the cleaning section, the more the rate of abrasion increases.
Finding 3. When the density of a toner image, formed on the photoconductor increases, (that is, when the amount of toner particles adhered on the photoconductor increases), the amount of not transferred toner particles increases.
Based on the relationship shown in Findings 1, 2 and 3, when the surface of the photoconductor is divided into plural areas in the lateral direction, being perpendicular to the moving direction of the photoconductor, the amount of abrasion in each area depends upon an average character coverage ratio of each area. That is, if a previous print carries a reflection density distribution of the images in the lateral direction (being a distribution of the average character coverage ratio), the amount of abrasion changes slightly in each area of the photoconductor, corresponding to the average character coverage ratio of each area.
When the original document, carrying the above-described reflection density distribution of the images in the lateral direction, is repeatedly printed on a large number of the recording sheets, said slight changes of the amount of abrasion accumulate, whereby an area of uneven thickness of the photoconductor increases, and thereby the uneven reflection density becomes visible. In a normal case, in an area exhibiting lower thickness of the photoconductor, the half tone image becomes darker, compared with an area exhibiting relatively greater thickness of the photoconductor.
Further, though the occurrence mechanism differs from the above-described uneven reflection density exhibiting streaks, a technology is offered to prevent the problem of an image quality, which occurs when the original document, carrying a white area or relatively low reflection density image, in the lateral direction perpendicular to the moving direction of the photoconductor, is repeatedly printed on the recording sheets.
Japanese Patent 3,835,503 concerns the problem of image quality, in which bleeding or blurring is generated on the printed images, due to materials which prevent formation of latent images, accumulating on the white areas or the low reflection density areas in the lateral direction of the surface of the photoconductor.
In said Japanese Patent, the image covering density, in an image area on an image carrier facing a transfer sheet, is calculated, whereby in order to supply toner particles to white areas, or to lateral areas of the low image covering density, corresponding to low reflection areas, a toner band (being a toner patch image) is formed at inter-image areas between the image areas, aligned in the moving direction of the surface of the image carrier. Due to a polishing function of the toner particles, at contact portions of the cleaning blade, at the toner band, the materials to prevent formation of the latent images do not accumulate on the surface of the image carrier, whereby image bleeding and blurring are prevented from occurring on the produced image.
That is, the image covering density, at each lateral area in the lateral direction perpendicular to the moving direction of the surface of the image carrier, is checked whether the image covering density exceeds a predetermined threshold value or not, after that, the toner particles are supplied to the lateral areas exhibiting the image covering density which is less than the threshold value.
Accordingly, due to the technology listed in the above Patent Document, the uneven reflection density exhibiting the streaks cannot be totally overcome, which occurs due to the difference of the average image covering density, compared between lateral areas distributed in the lateral direction, or which occurs due to white areas or low density areas. Further, no toner particles are supplied to lateral areas of the image carrier, onto which no transfer sheet is introduced. Accordingly, if original documents of a narrow width of images, are continuously printed, and then an original document of a greater width of images and carrying the half-tone images, is subsequently printed, an image, formed on the areas of the image carrier, at which no toner particles were supplied, exhibits lower density as longitudinal strips, compared to areas of the image carrier on which the transfer sheet was introduced, which is a new problem to be solved.
Unexamined Japanese Patent Application Publication 2002-328,496 relates to a problem being specific to a single component developer, in which when original documents, carrying a low character coverage ratio, are continuously printed, since very few toner particles fly to the photoconductor drum from a developing sleeve, the toner particles remain on the developer sleeve without fly, whereby due to said remaining toner particles, the density of the printed image is adversely lowered, or fog is generated on the printed image. To overcome these problems, a black even pattern is formed on areas of the photoconductor, corresponding to the areas exhibiting a character coverage ratio of less than a predetermined value, whereby the toner particles of the developer sleeve corresponding to said areas are selected to be conveyed onto the photoconductor, so that the toner particles on the developer sleeve of the developing device are refreshed.
Accordingly, due to reasons described below, the technology disclosed in Unexamined Japanese Patent Application Publication 2002-328,496, cannot prevent the uneven reflection density, visible as streaks.
In the technology disclosed in Unexamined Japanese Patent Application Publication 2002-328,496, the character coverage ratios of specific lateral areas across the total lateral area are calculated. However, since only a predetermined amount of toner particles are supplied to each specific lateral area, having a character coverage ratio being less than the predetermined value, any abrade gap is not closed on the surface of the photoconductor which has become worn, due to the difference of the densities in the areas, as far as the character coverage ratio is greater than the predetermined value. Accordingly, any uneven reflection density exhibited by streaks, which is caused by differences of the average image covering density in each lateral area on the lateral distribution, still exists adverse.